This is a stacking discount bundle, 3 games adds an additional 65% off up to 5 games adding an additional 75% off the listed discount. If you have Humble Choice, that stacks on top of the discounts too.

For example if you have 5 or more games (from this byob) in your cart
with the 20% HBC discount each game have a -80% discount, not only the 75%
(fyi 20% x 25% => 5%)